1. Technical Field
This invention relates to a humidity sensor device for detecting and determining moisture in the surrounding atmosphere and a method for preparing the same.
2. Background Art
Known humidity sensors designed to detect humidity through changes of electrical properties, typically electric resistance include those using electrolytes such as lithium chloride, metal oxides, and organic polymers as the humidity sensitive material.
However, the humidity sensors using electrolytes such as lithium chloride can measure only a narrow range of humidity and are less resistant to water in that their performance can be altered by dew condensation and wetting. The humidity sensors using metal oxides are resistant to water, but low sensitive. Because of the lack of long-term stability when used alone, they undesirably require a heat cleaning circuit which would add to the operating cost and make the sensor structure complex.
Among the humidity sensitive materials, organic polymers, especially polymeric electrolytes having quaternary ammonium salt have been widely used in commercial and industrial applications and so appreciated.
For example, Japanese Patent Publication (JP-B) No. 61-54176 discloses a humidity sensitive material comprising aggregates of latex particles formed of a copolymer between a hydrophobic monomer and an ionic or non-ionic hydrophilic monomer and having a hydrophilic surface layer. There are exemplified some cationic compounds having primary to quaternary ammonium salts.
JP-B 62-7976 discloses a humidity sensitive material in the form of a polymer which is obtained by polymerizing a compound containing 2-hydroxy-3-methacryloxypropyl-trimethylammonium chloride to a degree of polymerization of 1,000 to 10,000.
JP-B 2-24465 discloses the use as a humidity sensitive polymeric thin film of a thin film of an ionene polymer having the structural formula:
xe2x80x94(N+(R1)(R2)Xxe2x88x92-A-N+(R3)(R4)Xxe2x88x92xe2x80x94B)nxe2x80x94
wherein R1 to R4 are alkyl, Xxe2x88x92 is a halide ion, A and B each are xe2x80x94(CH2)m xe2x80x94 wherein mxe2x89xa72, or a thin film of a mixture of the polymer with another polymer such as polyvinyl pyrrolidone for the purposes of improving substrate adhesion and water resistance.
Humidity sensors using the polymeric electrolytes exemplified above as the humidity sensitive material, however, are still low in water resistance in that the polymeric electrolytes can be partially leached in a high humidity region, especially in a dew condensing atmosphere. They also suffer from a hysteresis phenomenon that they produce different outputs at the same humidity depending on whether the humidity is increasing or decreasing. In a low humidity region having a relative humidity (RH) of less than 10%, they have so high resistance values that practical humidity measurement is impossible.
JP-A 7-318526 discloses that water resistance is improved by introducing an unsaturated bond into the ionene polymer at either end and crosslinking with ultraviolet radiation. In this case, a very thin film provides satisfactory properties and water resistance. However, the small thickness means so small an absolute amount of the polymer that the film is vulnerable to a gas which is dissolved in water to form ions (e.g., Cl2, NOx, SOx). The problem is serious particularly in a commercial application.
The conductivity of a film having a thickness above a certain level is not substantially altered even when gas-origin ions are created in the surface layer. The polymer layer obtained by the above crosslinking method, however, undergoes substantial swell upon absorption of water, giving rise to serious problems of crack generation and separation from the substrate. Under such circumstances, the film must be used at a thickness of no more than 1 xcexcm.
A primary object of the present invention is to provide a humidity sensor device having a humidity sensitive thin film which is resistant to water, maintains effective, stable performance over a long time even in a dew condensing atmosphere, is resistant to solvents and less vulnerable to gases such as nitrogen oxides, sulfur oxide and chlorine, and produces accurate outputs in a stable manner over a wide humidity region, especially in a low humidity region; and a method for preparing the same. Another object is to provide a simple method for preparing such a humidity sensor device having improved properties as mentioned above.
These and other objects are achieved by the present invention which is defined below as (1) to (38).
(1) A humidity sensor device comprising an insulating substrate, a pair of opposed electrodes disposed on the substrate to define a gap therebetween, and a humidity sensitive thin film lying on the gap,
said humidity sensitive thin film comprising a copolymer of at least one monomer of the following formula (1) with at least one monomer of the following formula (2): 
wherein A11 is a divalent group, each of R11, R12, R13 and R14 which may be the same or different is an alkyl group, each of Y11 and Y12 which may be the same or different is a monovalent group terminated with an ethylenically unsaturated reactive group, any two or more of R11 to R14, Y11, Y12, A11 and portions thereof adjoining the nitrogen (N) atom may bond together to form a ring with the nitrogen (N) atom, and each of X11xe2x88x92 and X12xe2x88x92 which may be the same or different is an anion, 
wherein each of A and B is a divalent group, each of Y1, Y2, Y3, Y4, Y5 and Y6 which may be the same or different is a monovalent group, at least one of Y""s is a group terminated with an ethylenically unsaturated reactive group, any two or more of Y1, Y2, Y3, Y4, Y5, A and portions thereof adjoining the nitrogen (N) atom or any two or more of Y4, Y5, Y6, B and portions thereof adjoining the nitrogen (N) atom may bond together to form a ring with the nitrogen (N) atom, each of X1xe2x88x92 and X2xe2x88x92 which may be the same or different is an anion, and n is a number of 2 to 5,000.
(2) The humidity sensor device of above (1) wherein the anions represented by X11xe2x88x92 and X12xe2x88x92 in formula (2) and the anions represented by X11xe2x88x92 and X12xe2x88x92 in formula (1) are halide ions.
(3) The humidity sensor device of above (2) wherein chloride ions or bromide ions are contained as the halide ions.
(4) The humidity sensor device of above (1) wherein the divalent groups represented by A and B in formula (2) and the divalent group represented by A11 in formula (1) each are an alkylene, alkenylene or arylene group or a mixture thereof.
(5) The humidity sensor device of above (1) wherein the monovalent groups represented by Y11 and Y12 in formula (1) each are an alkylene acrylate or methacrylate group or alkylene acrylate or methacrylate amide group.
(6) The humidity sensor device of above (1) wherein the monomer of formula (1) is a difunctional monomer obtained by reacting a dialkylaminoethyl acrylate or methacrylate or a dialkylaminoethyl acrylate or methacrylate amide with a dihalogen compound.
(7) The humidity sensor device of above (1) wherein the monomer of formula (1) is obtained by reacting an acrylic unsaturated compound having a dialkylamino group with a dihalogen compound of the divalent group represented by A11 in formula (1).
(8) The humidity sensor device of above (1) wherein the copolymer further includes an acrylic monomer having an alkoxysilyl group.
(9) The humidity sensor device of above (1) wherein the insulating substrate is an insulating substrate from which contaminants and/or oxides on its uppermost surface layer have been removed by physical means.
(10) A method for preparing a humidity sensor device comprising an insulating substrate, a pair of opposed electrodes disposed on the substrate to define a gap therebetween, and a humidity sensitive thin film lying on the gap, said method comprising the steps of coating a coating solution containing a monomer of the following formula (1) and a monomer of the following formula (2) onto the insulating substrate and causing the monomers to copolymerize to form the humidity sensitive thin film, 
wherein A11 is a divalent group, each of R11, R12, R13 and R14 which may be the same or different is an alkyl group, each of Y11 and Y12 which may be the same or different is a monovalent group terminated with an ethylenically unsaturated reactive group, any two or more of R11 to R14, Y11, Y12, A11 and portions thereof adjoining the nitrogen (N) atom may bond together to form a ring with the nitrogen (N) atom, and each of X11xe2x88x92 and X12xe2x88x92 which may be the same or different is an anion, 
wherein each of A and B is a divalent group, each of Y1, Y2, Y3, Y4, Y5 and Y6 which may be the same or different is a monovalent group, at least one of Y""s is a group terminated with an ethylenically unsaturated reactive group, any two or more of Y1, Y2, Y3, Y4, Y5, A and portions thereof adjoining the nitrogen (N) atom or any two or more of Y4, Y5, Y6, B and portions thereof adjoining the nitrogen (N) atom may bond together to form a ring with the nitrogen (N) atom, each of X11xe2x88x92 and X12xe2x88x92 which may be the same or different is an anion, and n is a number of 2 to 5,000.
(11) The method for preparing a humidity sensor device of above (10) wherein the humidity sensitive thin film is formed by previously treating the insulating substrate with an acrylic monomer having an alkoxysilyl group for joining acrylic functional groups to the insulating substrate, then coating said coating solution, or by previously incorporating an acrylic monomer having an alkoxysilyl group into said coating solution.
(12) The method for preparing a humidity sensor device of above (10), further comprising the step of removing contaminants and/or oxides on the uppermost surface layer of the insulating substrate by physical means, prior to the step of coating said coating solution.
(13) The method for preparing a humidity sensor device of above (12) wherein said physical means is plasma surface treatment.
(14) The method for preparing a humidity sensor device of above (10) wherein the copolymerization is carried out by irradiation of ultraviolet radiation.
(15) A humidity sensor device comprising an insulating substrate, a pair of opposed electrodes disposed on the substrate to define a gap therebetween, and a humidity sensitive thin film lying on the gap,
said humidity sensitive thin film comprising a copolymer of a monomer of the following formula (1) with an acrylic monomer having an alkoxysilyl group, 
wherein A11 is a divalent group, each of R11, R12, R13 and R14 which may be the same or different is an alkyl group, each of Y11 and Y12 which may be the same or different is a monovalent group terminated with an unsaturated reactive group, any two or more of R11 to R14, Y11, Y12, A11 and portions thereof adjoining the nitrogen (N) atom may bond together to form a ring with the nitrogen (N) atom, and each of X11xe2x88x92 and X12xe2x88x92 which may be the same or different is an anion.
(16) The humidity sensor device of above (15) wherein the humidity sensitive thin film is secured to the substrate through reaction of alkoxysilyl groups in the copolymer in the humidity sensitive thin film with functional groups on the surface of the insulating substrate and/or the electrodes.
(17) The humidity sensor device of above (15) wherein at least 30 mol % of the anions represented by X11xe2x88x92 and X12xe2x88x92 in the copolymer are chloride ions.
(18) The humidity sensor device of above (15) wherein the divalent group represented by A11 in formula (1) is an alkylene, alkenylene or arylene group or a mixture thereof.
(19) The humidity sensor device of above (15) wherein the monovalent groups represented by Y11 and Y12 in formula (1) each are an alkylene acrylate or methacrylate group or alkylene acrylate or methacrylate amide group.
(20) The humidity sensor device of above (15) wherein the monomer of formula (1) is a difunctional monomer obtained by reacting a dialkylaminoethyl acrylate or methacrylate or a dialkylaminoethyl acrylate or methacrylate amide with a dihalogen compound.
(21) The humidity sensor device of above (15) wherein the monomer of formula (1) is obtained by reacting an acrylic unsaturated compound having a dialkylamino group with a dihalogen compound of the divalent group represented by A11 in formula (1).
(22) A method for preparing a humidity sensor device comprising an insulating substrate, a pair of opposed electrodes disposed on the substrate to define a gap therebetween, and a humidity sensitive thin film lying on the gap, said method comprising the steps of previously treating the insulating substrate with an acrylic monomer having an alkoxysilyl group for joining acrylic functional groups to the insulating substrate, coating a monomer of the following formula (1), then causing the monomer to polymerize to form the humidity sensitive thin film, 
wherein A11 is a divalent group, each of R11, R12, R13 and R14 which may be the same or different is an alkyl group, each of Y11 and Y12 which may be the same or different is a monovalent group terminated with an unsaturated reactive group, any two or more of R11 to R14, Y11, Y12, A11 and portions thereof adjoining the nitrogen (N) atom may bond together to form a ring with the nitrogen (N) atom, and each of X11xe2x88x92 and X12xe2x88x92 which may be the same or different is an anion.
(23) The method for preparing a humidity sensor device of above (22) wherein the polymerization is carried out by irradiation of ultraviolet radiation.
(24) A method for preparing a humidity sensor device comprising an insulating substrate, a pair of opposed electrodes disposed on the substrate to define a gap therebetween, and a humidity sensitive thin film lying on the gap, said method comprising the steps of coating a monomer of the following formula (1) and an acrylic monomer having an alkoxysilyl group onto the insulating substrate, causing the monomers to copolymerize into a copolymer, and causing the alkoxysilyl groups to react with functional groups on the surface of the insulating substrate in the presence of water vapor to secure the copolymer to the insulating substrate, for thereby forming the humidity sensitive thin film, 
wherein A11 is a divalent group, each of R11, R12, R13 and R14 which may be the same or different is an alkyl group, each of Y11 and Y12 which may be the same or different is a monovalent group terminated with an unsaturated reactive group, any two or more of R11 to R14, Y11, Y12, A11 and portions thereof adjoining the nitrogen (N) atom may bond together to form a ring with the nitrogen (N) atom, each of X11xe2x88x92 and X12xe2x88x92 which may be the same or different is an anion.
(25) The method for preparing a humidity sensor device of above (24) wherein the polymerization is carried out by irradiation of ultraviolet radiation.
(26) A humidity sensor device comprising an insulating substrate, a pair of opposed electrodes disposed on the substrate to define a gap therebetween, and a humidity sensitive thin film lying on the gap, wherein said insulating substrate is an insulating substrate from which contaminants and/or oxides on its uppermost surface layer have been removed by physical means, and said humidity sensitive thin film comprises a polymer resulting from a monomer of the following formula (1): 
wherein A11 is a divalent group, each of R11, R12, R13 and R14 which may be the same or different is an alkyl group, each of Y11 and Y12 which may be the same or different is a monovalent group terminated with an unsaturated reactive group, any two or more of R11 to R14, Y11, Y12, A11 and portions thereof adjoining the nitrogen (N) atom may bond together to form a ring with the nitrogen (N) atom, and each of X11 xe2x88x92 and X12xe2x88x92 which may be the same or different is an anion.
(27) The humidity sensor device of above (26) wherein at least 30 mol % of the anions represented by X11xe2x88x92 and X12xe2x88x92 in the copolymer are chloride ions.
(28) The humidity sensor device of above (26) wherein the divalent group represented by A11 in formula (1) is an alkylene, alkenylene or arylene group or a mixture thereof.
(29) The humidity sensor device of above (26) wherein the monovalent groups represented by Y11 and Y12 in formula (1) each are an alkylene acrylate or methacrylate group or alkylene acrylate or methacrylate amide group.
(30) The humidity sensor device of above (26) wherein the monomer of formula (1) is a difunctional monomer obtained by reacting a dialkylaminoethyl acrylate or methacrylate or a dialkylaminoethyl acrylate or methacrylate amide with a dihalogen compound.
(31) The humidity sensor device of above (26) wherein the monomer of formula (1) is obtained by reacting an acrylic unsaturated compound having a dialkylamino group with a dihalogen compound of the divalent group represented by A11 in formula (1).
(32) A method for preparing a humidity sensor device comprising an insulating substrate, a pair of opposed electrodes disposed on the substrate to define a gap therebetween, and a humidity sensitive thin film lying on the gap, said method comprising the steps of removing contaminants and/or oxides on the uppermost surface layer of the insulating substrate by physical means, then coating a monomer of the following formula (1) onto the insulating substrate, and causing the monomer to copolymerize on the insulating substrate for thereby forming the humidity sensitive thin film: 
wherein A11 is a divalent group, each of R11, R12, R13 and R14 which may be the same or different is an alkyl group, each of Y11 and Y12 which may be the same or different is a monovalent group terminated with an unsaturated reactive group, any two or more of R11 to R14, Y11, Y12, A11 and portions thereof adjoining the nitrogen (N) atom may bond together to form a ring with the nitrogen (N) atom, and each of X11xe2x88x92 and X12xe2x88x92 which may be the same or different is an anion.
(33) The method for preparing a humidity sensor device of above (32) wherein said physical means is plasma surface treatment.
(34) The method for preparing a humidity sensor device of above (32) wherein the polymerization is carried out by irradiation of ultraviolet radiation.
(35) A method for preparing a humidity sensor device comprising an insulating substrate, a pair of opposed electrodes disposed on the substrate to define a gap therebetween, and a humidity sensitive thin film lying on the gap, said method comprising the steps of removing contaminants and/or oxides on the uppermost surface layer of the insulating substrate by physical means, then coating a monomer having an ethylenically unsaturated reactive group onto the insulating substrate, and causing the monomer to copolymerize on the insulating substrate for thereby forming the humidity sensitive thin film.
(36) The method for preparing a humidity sensor device of above (35) wherein said physical means is plasma surface treatment.
(37) The method for preparing a humidity sensor device of above (35) wherein the monomer having an ethylenically unsaturated reactive group contains a quaternary ammonium salt.
(38) A humidity sensor device comprising an insulating substrate, a pair of opposed electrodes disposed on the substrate to define a gap therebetween, and a humidity sensitive thin film lying on the gap, wherein said the insulating substrate is an insulating substrate from which contaminants and/or oxides on its uppermost surface layer have been removed by physical means, and said humidity sensitive thin film comprises a polymer resulting from a monomer having an ethylenically unsaturated reactive group.
According to the invention, a humidity sensitive thin film of an electrically conductive polymer is formed so as to cover a pair of opposed electrodes on an insulating substrate.
In the first embodiment of the present invention, the humidity sensitive thin film is obtained by furnishing at least one of crosslinking monomers of formula (1) (abbreviated as monomer of formula (1), hereinafter) and at least one of crosslinking monomers of formula (2) (abbreviated as monomer of formula (2), hereinafter), that is, at least two monomers together, and causing the monomers to polymerize together by suitable means such as ultraviolet irradiation or heating. As seen from the monomers of formulae (1) and (2), the copolymer obtained therefrom is characterized by the possession of an ionene polymer structure having a quaternary ammonium salt (inclusive of cyclized one).
Since the monomers are preferably difunctional or have two acrylic unsaturated reactive groups, the crosslinking copolymer undergoes three-dimensional reaction to form a crosslinked structure which becomes insoluble in water.
In the humidity sensitive thin film using the crosslinking copolymer of ionene polymer structure, the quaternary ammonium salt moiety contained in the copolymer molecule contributes to electric conductivity and the counter ion thereto is dissociated with moisture in the surrounding atmosphere to develop ionic conduction. Humidity is detected by utilizing the phenomenon that the degree of dissociation varies as the moisture content in the atmosphere increases or decreases. In the first embodiment of the invention wherein the ionene polymer structure is included in the crosslinking copolymer, the copolymer is close to the backbone type with a less degree of freedom unlike the so-called pendant type. Its humidity response does not develop hysteresis.
The humidity sensitive film which is formed by copolymerization of monomers on the electrodes does not always have a uniform distribution of crosslinking sites within it because polymerization takes place in a dry state. This is because, when copolymerization is effected by exposure to UV radiation, for example, the difference of photon density between the surface and the interior of the film becomes large in the case of solids, with the tendency that an outer side has a higher crosslinking density. The humidity sensitive film is more likely to crack upon swelling due to water absorption as it becomes thicker. It is then necessary to make the crosslinking density uniform or to increase the number of crosslinking sites.
Therefore, the first embodiment of the invention is characterized in that the monomer of formula (1) which can assume an ionene polymer structure having improved properties when polymerized to form a humidity sensitive material is combined as the crosslinking agent with the monomer of formula (2), for thereby increasing the number of crosslinking sites while maintaining a low impedance. Since the monomer of formula (1) and the monomer of formula (2), which differ in molecular weight, are analogous monomers in that they can possess an ionene polymer structure within them, they enable to increase the number of crosslinking sites without reducing the ion density.
From the standpoint of device design, by changing the molecular weight and mixing ratio of the monomer of formula (2), the factor of greater interest can be selected among basic factors of the humidity sensor including water resistance, gas resistance and hysteresis, which enables free design. An increase of ion density improves gas resistance, and an increase of crosslinking density improves water resistance. As the molecular weight increases, the monomer of formula (2) becomes more flexible, which restrains cracking after swelling. It is noted that to ensure water resistance in this case, the content of the monomer of formula (1) must be increased to enhance the degree of crosslinking. On the other hand, extreme crosslinking increases the possibility of hysteresis, which necessitates appropriate adjustment.
Referring to formulae (1) and (2), the structural factors that dictate the conductivity of the crosslinking copolymer include the number of carbon atoms in the backbone of A and B in formula (2) and A11 in formula (1) (i.e., the length of divalent linking group), the type of anions X1xe2x88x92 and X2xe2x88x92 in formula (2), and X11xe2x88x92 and X12xe2x88x92 in formula (1). Therefore, a sensor device having desired properties can also be designed by combining the monomers in which these parameters are suitable selected.
In contrast, with respect to crosslinking agents which are commonly added in order to increase the number of crosslinking sites, such as divinyl benzene, it is confirmed that the strength of the film and the number of crosslinking sites are increased by adding them to the monomer of formula (2). However, such a customary crosslinking agent results in a humidity sensitive film which has an increased impedance because its swelling is suppressed due to the increased number of crosslinking sites by the addition of the crosslinking agent and because the crosslinking agent itself lacks electric conductivity. It is not suitable as the humidity sensitive film for humidity sensors.
JP-B 2-24465 cited above discloses a polymer containing a quaternary ammonium salt in its backbone, which is similar to the polymer according to the first embodiment of the invention. Unlike the first embodiment of the invention, the formation of a crosslinking structure within the polymer is referred to nowhere. In this respect, the first embodiment of the present invention is clearly different in construction and features from the patent publication. Although the combined use of another polymer such as polyvinyl pyrrolidone for the purpose of improving water resistance is recommended in the patent publication, the water resistance is apparently inferior.
Likewise, in JP-A 4-309855 and JP-A 7-128271, a methacryloxypropyl trimethyl ammonium salt having a hydroxyl group and trimethylolpropane trimethacrylate are applied to form a coating, after which the coating is polymerized and insolubilized for allegedly improving water resistance. What is disclosed is a method of mixing two or more polymers, followed by polymerization. Since the ammonium group on the humidity sensitive material is the so-called pendant type, the humidity response develops substantial hysteresis.
In the second embodiment of the present invention, the humidity sensitive thin film contains a crosslinking copolymer of a monomer of formula (1) with an acrylic monomer having an alkoxysilyl group. Then the humidity sensitive thin film is joined to the insulating substrate through reaction with the silane coupling agent within the molecule. That is, the humidity sensitive thin film is obtained by coating an aqueous solution or organic solvent solution of the monomer and the silane coupling agent having an acrylic unsaturated bond, drying, and subjecting the monomer and the silane coupling agent to copolymerization and crosslinking by irradiation of UV radiation and/or heating. The copolymer is characterized by having an ionene polymer structure having a quaternary ammonium salt (including cyclized one) as seen from the monomer of formula (1) and the silane coupling agent within the molecule.
In the humidity sensitive thin film using the crosslinking copolymer of ionene polymer structure, the quaternary ammonium salt moiety contained in the copolymer molecule contributes to electric conductivity and the counter ion thereto is dissociated with moisture in the surrounding atmosphere to develop ionic conduction. Humidity is detected by utilizing the phenomenon that the degree of dissociation varies as the moisture content in the atmosphere increases or decreases. In the second embodiment of the invention wherein the ionene polymer structure is included in the crosslinking copolymer, the copolymer is close to the backbone type with a less degree of freedom unlike the so-called pendant type. Its humidity response does not develop hysteresis.
In the invention, the ion density and crosslinking degree of the humidity sensitive film can be controlled by adjusting the length of divalent linking group A11. This leads to the design that permits the factor of greater interest to be selected among factors including water resistance, gas resistance and hysteresis, which enables free design. An increase of ion density improves gas resistance, and an increase of crosslinking density improves water resistance. On the other hand, extreme crosslinking increases the possibility of hysteresis.
Since the monomers are preferably difunctional or have two acrylic unsaturated reactive groups, the crosslinking copolymer undergoes three-dimensional reaction to form a crosslinked structure which becomes insoluble in water. Since the silane coupling agent structure is included within the molecule, the copolymer can be firmly joined to the substrate through covalent bonds by reacting with functional groups such as hydroxyl groups on the substrate surface and/or electrode surface. These bonds ensure that the humidity sensitive thin film remains water resistant without becoming loose or peeling even when it is thick. The humidity sensor has improved water resistance and gas resistance as well as improved solvent resistance.
In the embodiment wherein the counter ion to the quaternary ammonium salt group in the copolymer is a chloride ion, a low humidity region of RH 10% or lower can be measured, substantially spreading the measurable humidity region. The humidity sensor of the invention can measure humidity over the entire range from RH 0% to RH 100%, which was unmeasurable with conventional sensors.
JP-B 2-24465 cited above discloses a polymer containing a quaternary ammonium salt in its backbone, which is similar to the polymer according to the second embodiment of the invention. Unlike the present invention, the structure having the silane coupling agent incorporated within the polymer and the crosslinking structure are referred to nowhere. Therefore, the second embodiment of the present invention is clearly different in construction and features from the patent publication. Although the combined use of another polymer such as polyvinyl pyrrolidone for the purpose of improving water resistance is recommended in the patent publication, the water resistance is apparently inferior.
Also, JP-A 4-258750 intends to improve water resistance by using an acrylic polymer having a cationic radical and a hydroxyl group and a polyfunctional isocyanate compound to form a three-dimensionally crosslinked polymer. With this method, it is expected that some of isocyanate is joined to the substrate surface by reacting with hydroxyl groups on the substrate surface, but its effect is little.
Likewise, in JP-A 4-309855 and JP-A 7-128271, a methacryloxypropyl trimethyl ammonium salt having a hydroxyl group and trimethylolpropane trimethacrylate are applied to form a coating, after which the coating is polymerized and insolubilized for allegedly improving water resistance. However, this fails to achieve a substantial improvement in the adhesion to the substrate.
Furthermore, JP-A 7-318525 discloses a bonding method involving treating a substrate with silicon tetrachloride, and reacting with poly(2-hydroxy-3-methacryloxypropyl) trimethyl ammonium salt for thereby reacting the substrate surface with hydroxyl groups in the polymer. This method establishes the adhesion of the humidity sensitive film to the substrate, but encounters difficulty in reproducing substrate treating conditions because of the high reactivity and storage instability of silicon tetrachloride. Also, since the ammonium group of the humidity sensitive material is of the so-called pendant type, it is unavoidable that humidity response develops hysteresis.
In the third embodiment of the present invention wherein contaminants and/or oxides on the uppermost surface layer of the insulating substrate are removed by physical means before a humidity sensitive thin film is formed, the humidity sensitive thin film is comprised of a polymer obtained from a monomer of formula (1), obviating the use of a monomer of formula (2) and an acrylic monomer having an alkoxysilyl group.
The insulating substrate on which the humidity sensitive film is to be formed is, most often, an alumina substrate whose uppermost surface layer is covered with contaminants, adsorbed gases, organic matter and oxides of the substrate itself, which preclude the humidity sensitive film from adhering thereto. The removal of contaminants improves the adhesion of the humidity sensitive thin film to the insulating substrate.
The reason why treatment is done by physical means is that treatment by chemical means, which can remove contaminants and organic matter, is not preferable because of difficulty to remove the oxide layer and the problem of used solution at the end of chemical treatment. In contrast, the treatment by physical means can remove contaminants and oxides in a relatively simple manner while minimizing damage to the underlay. The material which is treated by physical means may be selected from a wide variety.
Among several physical means, plasma surface treatment is highly effective and preferable. The plasma surface treatment (to be referred to as xe2x80x9cplasma treatment,xe2x80x9d hereinafter) means that active electrons, radicals, ions and molecules in a plasma interact with the insulating substrate surface to exert etching and implantation effects for thereby modifying the surface state.
The plasma to be generated varies with the plasma generating atmosphere and is generally divided into oxygen plasma and hydrogen plasma depending on the gas species used. Use of both oxygen plasma and hydrogen plasma is preferred. Oxygen plasma is effective for removal of surface organic contaminants and surface modification, and hydrogen plasma is able to reduce metal oxides.
In the third embodiment of the invention wherein contaminants and/or oxides on the uppermost surface layer of the insulating substrate are removed by plasma-assisted physical means, the adhesion between the humidity sensitive thin film and the insulating substrate is presumably enhanced by the two mechanisms described below.
First, contaminants and oxides on the uppermost surface layer serve as a weak xe2x80x9cboundary layerxe2x80x9d at the interface between the humidity sensitive thin film and the insulating substrate. The adhesion is strengthened by removal of this boundary layer.
Secondly, the removal of contaminants and/or oxides on the uppermost surface layer renders the insulating substrate surface more wettable so that upon application of the humidity sensitive thin film-forming monomer, the monomer can reach and penetrate into pores in the insulating substrate, and after polymerization, the humidity sensitive thin film is bound and rooted to the insulating substrate to provide the xe2x80x9canchorxe2x80x9d effect of tightly bonding the humidity sensitive thin film to the insulating substrate.
In the third embodiment of the invention, it is believed that the latter effect provides a predominant contribution.
The method of the invention according to the third embodiment wherein contaminants and/or oxides on the uppermost surface layer of the insulating substrate are removed by physical means before a humidity sensitive thin film is formed is applicable to not only the humidity sensitive thin film which is comprised of a polymer obtained from a monomer of formula (1), but also the humidity sensitive thin film which is comprised of a polymer obtained from a monomer having an ethylenically unsaturated reactive group, preferably a monomer having an ethylenically unsaturated reactive group and a quaternary ammonium salt. In either case, the adhesion between the humidity sensitive thin film and the insulating substrate is improved. It is noted that the quaternary ammonium salts include, in a narrow sense, quaternary ammonium salts which are ammonium salts having four alkyl groups bonded to a nitrogen atom and, in a broad sense, ammonium type salts resulting from primary to tertiary amines.